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γ-Al2O3–Ce catalysts for benzene combustion
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Catalysis Today 133–135 (2008) 541–547 www.elsevier.com/locate/cattod
Improved Pd/g-Al2O3–Ce catalysts for benzene combustion J.M. Padilla a, G. Del Angel a,*, J. Navarrete b Universidad Auto´noma Metropolitana, Iztapalapa, Departamento de Quı´mica, A´rea de Cata´lisis, Av. San Rafael Atlixco No. 186, C.P. 09340, AP. 55-534, Me´xico, D.F., Mexico b Instituto Mexicano del Petro´leo, Programa de Ingenierı´a Molecular, Me´xico, D.F. 07730, Mexico
a
Available online 1 February 2008
Abstract The catalytic oxidation of benzene on Pd (0.5 wt%)/g-Al2O3–Ce catalysts with 1, 5 and 10 wt% Ce was investigated. The alumina–Ce supports were prepared by adding a Ce-nitrate precursor to an aluminum boehmite. The presence of Lewis acid sites was determined by FTIR-pyridine adsorption spectroscopy. By FTIR-CO adsorption, it was found that the presence of Ce leads to the stabilization of PdO on the supports. The presence of the PdO species on the Pd/g-Al2O3–Ce catalysts was confirmed by X-ray photoelectron spectroscopy (XPS). The combustion of benzene (2600 ppm) in air was carried out at atmospheric pressure. Two behaviors were observed: for the catalysts with low Ce content (1%) and without Ce, the combustion occurs in a broad temperature range 100–280 8C, meanwhile, catalysts with high Ce contents (5, 10%), the benzene combustion occurs in the range 220–280 8C. An activation treatment on Pd/g-Al2O3–Ce catalysts with a reactant mixture at 250 8C for 20 h reduces the temperature of the total combustion to 250 8C. A better stability on Pd/g-Al2O3–Ce catalysts containing 5 and 10% Ce was observed. These results suggest that the oxidative catalytic properties of cerium oxide are shown at temperatures higher than 220 8C, and that an activation treatment is needed to obtain an optimal Pd8/PdO ratio for these species. The stability showed by the catalysts is explained by the effect of the CeO2, which inhibits the deposition of carbonaceous species on the Pd surface. # 2007 Elsevier B.V. All rights reserved. Keywords: Combustion; VOC; Benzene combustion; Pd/g-Al2O3–Ce catalysts; XPS-Pd spectroscopy; FTIR-CO adsorption; FTIR-pyridine adsorption
1. Introduction Air pollution has become one of the most complex environmental problems. Among the most common contamination sources is that produced by volatile organic compounds (VOC). Benzene is one of the main pollutants whose principal sources are petrochemical industries and vehicles [1–4]. The oxidation of the VOCs on a catalyst takes place at temperatures which are lower than those required for thermal destruction [2]. Noble metals such as Pt, Rh and Pd, deposited in conventional supports such as Al2O3, SiO2, etc., have shown to be efficient for this reaction [4]. The Al2O3/CeO2 support is one of the most widely used in three-way automotive catalysts (used for the elimination of the pollutant gases in the exhaust automobile gases). The cerium addition to alumina produces structural changes, improves the dispersion of the metal and it participates
in the stabilization of alumina, avoiding the thermal sinterization. On the other hand, the Ce3+/Ce4+ oxidation–reduction properties make cerium a good promoter for the combustion reaction due to its capacity to deliver oxygen. [5]. The present work is a study of the effect of Ce content on Pd/ g-Al2O3–Ce catalysts for the benzene combustion, as well as the effect of the activation and stabilization of the catalysts after 30 h on stream. The g-Al2O3–Ce supports were obtained from the addition of cerium nitrate to an aluminum boehmite and the characterizations were carried out by BET nitrogen adsorption, Fourier transform infrared spectroscopy of pyridine (FTIR), FTIR-CO adsorption and X-ray photoelectron spectroscopy (XPS). 2. Experimental 2.1. Support preparations
* Corresponding author. E-mail address: [email protected] (G. Del Angel). 0920-5861/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2007.12.053
The g-Al2O3 reference support was obtained by calcination in air of the aluminum boehmite phase (Catapal-B Condea high
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purity 99.999%) at 650 8C. The alumina doped with Ce with 1, 5 and 10 wt%, was prepared by adding Ce(NO3)36H2O (Strem Chemicals 99.9%) to the aluminum boehmite which were maintained in stirring in an aqueous media for 4 h, the solids were dried in an oven at 120 8C for 12 h and then calcined in air flow at 650 8C for 24 h. The supports were labeled as g-Al2O3– CeX, where X represents the wt% of Ce in the support.
equipped with CaF2 windows. The samples were pretreated in situ at 400 8C for 30 min under vacuum (10 6 Torr). The CO admission at 20 Torr was carried out at 200 8C, afterwards the cell was cooled down to room temperature, the excess of CO was evacuated under vacuum during 10 min and then the CO absorption spectra were recorded. 2.7. X-ray photoelectron spectroscopy
2.2. Catalysts preparation The Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts were prepared by impregnation of the supports with a PdCl2 (Strem Chemicals 99.9%) aqueous solution at pH 2 to obtain 0.5 wt% of Pd in all the catalysts. Afterwards the catalysts were calcined at 500 8C during 4 h and then reduced with hydrogen at 500 8C for 5 h. 2.3. Specific surface area, BET The determination of the BET surface area and the pore diameter was carried out in a QuantaChrome Multistation Autosorb 3B analyzer. Before performing the adsorption measurements, the samples were outgassed at 400 8C, in a vacuum of 10 3 Torr for 3 h. The specific surface area was calculated by the BET equation and the pore diameter by the BJH method. 2.4. TPO measurements The TPO study was carried out in a CHEMBET-3000 apparatus using a thermal conductivity detector (TCD), and 0.1 g of catalyst. In these experiments, the flow rate of the 5% O2/95% He mixture was 10 mL/min and the heating rate was 10 8C/min. Finally, the spectra were recorded, for fresh and after-combustion samples, from room temperature to 500 8C, only one peak was observed at around 100 8C. 2.5. FTIR-pyridine adsorption The FTIR spectra for pyridine adsorption were determined with a Nicolet model 170-SX spectrometer. The powdered sample was pressed until a transparent sample was obtained, which was placed in a glass cell equipped with CaF2 windows. The samples were pretreated in a vacuum of 10 6 Torr at 400 8C for 30 min in order to clean the surface. The samples were then cooled down to room temperature. The pyridine contained in a glass bulb coupled to the cell was passed on the catalyst. After the pyridine excess was eliminated under vacuum for 10 min, and then the spectra were registered at different desorption temperatures. The pyridine desorbed at each temperature was eliminated under vacuum (10 6 Torr).
The XPS analyses were carried out in a THERMO VG ESCALAB 250 spectrometer equipped with an aluminum anode (energy of 1486.8 eV) and with an X-ray system monocromator. The X-ray source was powered at 15 kV and 7.5 mA. In order to correct the effect of charge on the XPS spectra, all binding energies were referenced to the C 1s line of adventitious carbon at 284.6 eV. The reduced samples were placed on a thin sheet of indium and then analyzed. In order to control the sample charge in all experiments, an electron flood gun was used. 2.8. Catalytic activity measurements The combustion of benzene in vapor phase was carried out in a conventional flow U-shape glass reactor used in the differential mode at atmospheric pressure. The 2600 ppm of benzene in air were used as reactants in a total flow of 90 cm3/ min. The catalyst (0.1 g) was first treated under air flow (90 cm3/min) at 500 8C for 45 min, afterwards the catalytic system was treated under a flow of a mixture of benzene (2600 ppm)/air at 500 8C during 15 min. Then, the temperature was diminished to 100 8C, under the reactant mixture flow, at this moment the combustion reaction was carried out at different temperatures: 100, 140, 180, 220, 250 and 280 8C. The activity tests were done once a steady state was reached for each temperature. The conversions were measured during this period, where, in the end, almost constant values were reached. To measure the effect of the activation treatment on the catalyst conversions, at the different temperatures, we proceeded in the following way: the sample was treated under a benzene (2600 ppm)/air flow at 250 8C and then maintained on stream during 20 h. Afterwards the combustion reaction proceeded at 100 8C and the temperature was increased by steps of 50 8C until 250 8C. Reactants and products were analyzed by using an on-line gas chromatograph VARIAN 3400 CX equipped with a TCD (thermal conductivity detector) and a FID (flame ionization detector) using a J & W Scientific Megaboro 30 m, with a phase GSQ column that allowed measurements of CO2 and H2O concentrations and a PONA of 50 m columns to follow Benzene conversion. The products detected by the HP-GC–MS 5973 gas chromatograph mass spectrometer were, only, CO2 and H2O. 3. Results and discussion
2.6. FTIR-CO adsorption 3.1. Specific surface area The study of the FTIR-CO adsorption was carried out at room temperature using a Nicolet-FX710 apparatus. The samples pressed in thin wafers were placed in a Pyrex glass cell
The specific surface area of g-Al2O3 obtained from the calcination of the boehmite at 650 8C was around 210 m2/g.
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Table 1 Characterization of the Pd/g-Al2O3 and Pd/g-Al2O3–Ce Catalysts Catalyst
Pd (wt%)
Dispersion (%)
´˚ Particle size (A )
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
0.51 0.49 0.52 0.50
85 91 83 68
12.4 11.6 12.8 15.6
The addition of Ce produced a decrease in the specific surface area. The supports showed specific areas of 153, 145 and 146 m2/g, for g-Al2O3–Ce1, g-Al2O3–Ce5 g-Al2O3–Ce10, respectively. On the other hand, the pore diameter was ˚ ) in all the supports. The lower practically constant (78–94 A specific surface areas of the supports containing cerium indicate that probably the ceria crystallites plugged the aperture of the pores of alumina. However, the formation of Al–O–Ce– bonds cannot be discarded [6]. The content of Pd was determined by atomic absorption spectroscopy and the obtained values are reported in Table 1. The palladium content for all the catalysts varied between 0.49– 0.52 wt%, which was close to the Pd nominal (0.5 wt% Pd). 3.2. FTIR-pyridine adsorption The infrared spectra for the pyridine adsorption showed typical vibration bands of Lewis acid sites at 1590, 1490 and 1445 cm 1 in all the supports. In Fig. 1 are shown the mmol/g of pyridine adsorbed as a function of the desorption temperature for the different catalysts. It is observed a decrease in the number and strength of the Lewis acid sites with the cerium content in the support. The decrease of the g-alumina acidity with cerium addition can be explained by the deposition of Ce particles on the alumina acid sites. 3.3. FTIR-CO adsorption In Fig. 2 is shown the FTIR spectra of the CO adsorption for the different catalysts. The adsorption of CO on the Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts showed the typical absorption bands for CO adsorbed on Pd catalysts [7]. The adsorption band located at 1969 cm 1 in the Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts, has been assigned to symmetric and non-symmetric
Fig. 1. Pyridine adsorbed (mmol/g) as a function of the desorption temperature for the catalysts at different cerium contents (0, 1, 5 and 10%) obtained by FTIR.
Fig. 2. FTIR spectra of CO adsorbed on Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts.
adsorptions of CO in the regions 1910–1980 cm 1 [7]. For catalysts containing 5 and 10% Ce the band was slightly shifted to higher frequencies. On the other hand, the adsorption at around 2073 cm 1 also appeared shifted to higher frequencies in catalysts with 5 and 10% of Ce. The band in the 2085– 2065 cm 1 is assigned to the CO lineal adsorption on the high index crystallographic planes, i.e. adsorption on edges, corner, terraces and kinks [8]. The adsorption band at 2160 cm 1 in catalysts containing 5 and 10 wt% Ce was assigned to the CO– Pd2+ [9]. Palladium in its oxidized state (Pd2+) was only detectable in such samples. Then we can assume that the presence of cerium stabilizes the oxidized PdO phase. 3.4. Dispersion determined by FTIR-CO In this work, information was obtained from the elemental peaks of the CO spectra which were obtained from their deconvolution and the attribution of the vibration bands to determine the dispersion of the Pd catalysts [10,11], similarly, it has been reported in previous works [12]. The deconvolution of the FTIR spectra of the Pd/gAl2O3 catalysts in the elemental bands is shown in Fig. 3. The (a) and (b) peaks corresponded to the regions of 2050–2100 cm 1 and 2030–2050 cm 1 which are, respectively attributed to the discontinuity between the
Fig. 3. Deconvoluted FTIR-CO adsorbed spectrum for the Pd/g-Al2O3 catalyst.
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Fig. 4. Pd 3d5/2 and 3d3/2 core level spectra of Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts.
plans (edges, corner, terraces and kinks) where CO is linearly adsorbed. The (c) peak at 1950–1990 cm 1 was assigned to the 100 planes with bridged CO adsorbed, the domain of 1910– 1940 cm 1 corresponds to the planes 1 1 1 and also to bridged CO adsorbed (d), whereas the peaks in the ranges of 1840– 1880 cm 1 and 1810–1940 cm 1 (d and e) were associated to the interactions metal–support or the interfaces of the metallic support [13]. The dispersion values calculated from the deconvolution of the FTIR-CO adsorption spectra for all the catalysts are reported in Table 1. By the metal dispersion, defined as D (%) = N(CO)surf/N(Pd)total 100 and applying the Beer–Lambert law, we calculated the total number of CO molecules adsorbed on the surface by N(CO)surf = AiS/We, where Ai = integrated absorbance of the total bands obtained by the IR spectra, S = area of the wafer used in IR experiment, W = weight of the catalyst wafer and e = molar extinction coefficient (total) of CO adsorbed for Pd: 2.9 10 7 cm/mol [14]. For the Pd/g-Al2O3 catalyst, the dispersion was 85%, while for the Pd catalysts with 1, 5 and 10 wt% Ce, the dispersions were 91, 83 and 68%, respectively. The catalyst with 1% Ce showed the highest dispersions, this results are in agreement with those reported in the literature, where is wellknown that the presence of Ce at low concentrations favors the dispersion of the metal on Al2O3–CeO2 or La2O3–Al2O3 supports. In the Pd/g-Al2O3–Ce10 catalyst with high load of Ce (10%), the dispersion decreases giving a value of 68%. This dispersion diminution could be explained by the deposition of
cerium oxide on the surface of the metal, blocking some Pd sites [15]. 3.5. XPS spectra characterization The deconvoluted XPS photoelectron spectra for the Pd 3d core level region of the Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts are shown in Fig. 4. The deconvolution of the peaks showed three components for 3d5/2 and for 3d3/2 core levels in Pd/g-Al2O3–Ce catalysts. The binding energies of Pd 3d5/2, Al 2p core levels and the percent of Pd species are reported in Table 2. The binding energy of the Pd 3d5/2 core level for Pd in the pattern catalysts, Pd/g-Al2O3, is 335.7 eV, which corresponds to Pd8 [16]. The peak placed at 337.9 eV in the pattern catalyst Pd/g-Al2O3 has been related to Pd-chlorine species indicating that Pd is in a higher oxidation state likely Pd2+ [17]. In the catalysts containing cerium, the PdO species was observed at 3d5/2 core level with binding energies in the range of 336.3–336.0 eV for all the catalysts. As it can be seen, the Pd8 is present in all the Pd catalysts, however, the signal at 337.9 eV is only observed in the catalyst pattern, whose relative percent of this species, calculated from the deconvoluted peak, is around 21% (Table 2). The PdO was observed in catalysts where Ce was added: in catalysts with 1% Ce this species had 7%, for higher contents of Ce, 5 and 10%, the relative percent of this specie was 50 and 53%, respectively, Table 2. The binding energies corresponding to 337.1–337.3 eV, for Pd/g-Al2O3–Ce, have been assigned to the interaction of small Pd clusters with the support. The interaction of small clusters
J.M. Padilla et al. / Catalysis Today 133–135 (2008) 541–547 Table 2 Data from the XPS analyses of the Pd/g-Al2O3–Ce catalysts Catalyst
Binding energy (eV)
Pd species (%)
Al 2p
Pd/g-Al2O3
335.7 336.1 337.1 337.9
74.5
41 (Pd8) 7 (PdO) 31(Pd/Al2O3) 21 (PdCl2)
Pd/g-Al2O3–Ce1
335.6 336.3 337.2
74.5
32 (Pd8) 28 (PdO) 40 (Pd/Al2O3–Ce, Pd/Al2O3)
335.1 336.0 337.3
74.5
335.2 336.0 337.3
74.5
Pd/g-Al2O3–Ce5
Pd/g-Al2O3–Ce10
Table 3 Atomic percent of the different elements of the Pd/Al2O3-Ce catalysts determined by XPS Catalyst
Pd 3d5/2
34 (Pd8) 50 (PdO) 16 (Pd/Al2O3–Ce) 28 (Pd8) 53 (PdO) 19 (Pd/Al2O3–Ce)
with oxide substrates such as Pd–Al2O3 [18] and Pd–CeO2 has been studied [19], the results showed a shift to a higher binding energy in the Pd–g-Al2O3 and Pd–Ce systems. The typical Ce 3d region spectrum for the Pd/g-Al2O3–Ce10 catalyst is shown in Fig. 5. The characteristic peaks for Ce2O3 and CeO2 with binding energies of 880 and 882.6 eV for Ce3+ and Ce4+ species, respectively are observed. It was also observed a peak which has been associated to the interaction of cerium and palladium CePd3 with the corresponding energy of 884.4 eV [20]. The characteristic Al 2p binding energy of 74.5 eV is observed in all the samples, Table 2. The atomic percent of the different species and the surface atomic ratios calculated for Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts determined by XPS are reported in Tables 3 and 4, respectively. From this information, it is observed that the Ce/Al ratio increased and the Pd/Ce ratio decreased, this suggests that cerium could be segregated on the surface as the cerium content increases. The Cl/Pd ratio is around 2 in Pd/Al2O3 catalyst, which confirms the presence of the PdCl2 in this catalyst, that ratio decreases with the amount of Ce in the catalyst. The Cl/Ce ratio follows the same trend, also decreases with the cerium
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Atomic%
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
Al
O
Pd
Cl
Ce
41.33 42.30 41.95 41.96
58.45 57.56 57.12 57.13
0.07 0.08 0.16 0.15
0.15 0.06 0.05 0.05
– 0.20 0.71 0.79
Table 4 XPS atomic rations for the Pd/g-Al2O3–Ce catalysts Catalyst
Pd/Al
Ce/Al
Pd/Ce
Cl/Pd
Cl/Ce
Cl/Al
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
0.0017 0.0019 0.0038 0.0036
– 0.0047 0.0169 0.0188
– 0.4000 0.2254 0.1899
2.1429 0.7500 0.3125 0.3333
– 0.3000 0.0704 0.0633
0.0036 0.0014 0.0012 0.0012
load. The Pd/Al2O3 and Pd/Al2O3–Ce1 catalysts show the highest Cl/Al ratios. 3.6. Catalytic activity The catalytic behavior of the Pd/g-Al2O3 and Pd/g-Al2O3– Ce catalysts in the combustion of benzene as a function of the temperature is reported in Table 5. Two different behaviors can be observed, the first presented by the Pd/g-Al2O3, Pd/gAl2O3–Ce1 catalysts and the second by the Pd/g-Al2O3–Ce5 and Pd/g-Al2O3–Ce10 catalysts. In the Pd/alumina catalyst without cerium and with 1% Ce, the benzene combustion occurred in a wide range of temperatures starting from 100 8C and rising to the total conversion, towards CO2, at 280 8C. On the other hand, the catalysts containing 5 and 10% Ce, the combustion of benzene was produced in a low interval of temperatures, 250–280 8C, with selectivity towards CO2 of 100%. We assume that two different mechanisms of benzene oxidation occurred in both cases. In the first case, the behavior of the Pd/g-Al2O3 and Pd/gAl2O3–Ce1 catalysts can be explained by the acidic character of the alumina support. According to the literature the catalysts with supports showing high acidity present a good performance for the combustion reactions [21]. It has been reported that the organic compounds interact with the strong Lewis acid sites which activate the organic compounds [22]. We can assume that both catalysts, without Ce and containing 1% Ce have enough Table 5 Benzene combustion on Pd/g-Al2O3–Ce catalysts with 0, 1, 5, 10 wt% Ce Catalyst
Fig. 5. XPS spectra at Ce 3d region of the Pd/g-Al2O3–Ce10 catalyst, showing the deconvoluted peaks.
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
Combustion of benzene (%) 100 8C
140 8C
180 8C
220 8C
250 8C
280 8C
38 31 0 0
74 35 0 0
78 75 0 0
85 88 0 0
90 97 99 82
100 100 100 100
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acid sites (Fig. 1) in the catalyst to lead the combustion. Another factor that could be playing a role is the metal dispersion. The sensitive structure of the benzene combustion has been pointed out by various authors [23,3]. The Pd/g-Al2O3 and Pd/g-Al2O3– Ce1 catalysts showed a high dispersion of palladium (85 and 91%), then, it cannot be discarded a positive effect because of the small metal particles present in this catalysts. On the other hand, the Pd/g-Al2O3–Ce5 and Pd/g-Al2O3– Ce10 catalysts did not show activity in the range of temperatures of 100–220 8C, however, a shot in the conversion of benzene at 250 8C is observed, with values of 99 and 82%, respectively. In both catalysts the total benzene conversion was obtained at 280 8C. These results can be explained by a promotion effect of the cerium oxide which is reveled only at high temperatures. In the present case, we assume that the presence of Ce at 5 and 10% favors the oxidizing state of Pd to PdO on the support. Evidence of PdO in these catalysts was shown by infrared spectroscopy CO adsorption (Fig. 2) and by XPS results (Fig. 4). It has also been reported that the maximum in the catalytic activity in the combustion is reached when an optimal Pd8/PdO ratio is reached [24]. The no activity showed by the catalysts with 5 and 10% Ce at low temperatures could be due to the fact that cerium requires a high temperature or a long activation time to reach this ratio. 3.7. Activation effect on the catalytic activity In order to study the effect of the activation of the catalysts on the conversion of the benzene combustion, the catalysts were thermally treated with a reactant mixture. The catalysts were maintained on stream under a benzene (2600 ppm)/air flow at 250 8C during 20 h. After this, the benzene combustion was carried out from 100 to 250 8C. The results of the conversion of benzene at different temperatures are reported in Table 6. Combustion is observed in all the catalysts with and without cerium, beginning from 100 8C. This conversion is increased with the increment of the temperature, being slightly higher in the catalysts with cerium. In the Pd/g-Al2O3–Ce catalysts, no effect of the cerium concentration was observed on the conversion, which is very similar for the different samples, at the different temperatures. In the catalysts containing cerium the total conversion was obtained at 250 8C while in the pattern catalyst, Pd/g-Al2O3, the conversion at this temperature was 94%. As it can be observed, the effect of the activation on the catalysts with cerium leads to the activation of the species Pd8/ PdO which allows conversion at lower temperatures.
Fig. 6. Conversion of benzene after 30 h of reaction at 250 8C on Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts.
3.8. Catalytic stability In addition, the presence of cerium could play an important role in the stabilization of the Pd catalysts. In line with the aforementioned, a study of the stability of the catalysts was carried out in samples without activation treatment. The catalysts were maintained under the combustion reaction at 250 8C during 30 h, see Fig. 6. Again, two types of behavior were observed. For the Pd/g-Al2O3 and Pd/g-Al2O3–Ce1 catalysts a decrease in the conversion was observed after 20 h of reaction, whereas in the catalysts Pd/g-Al2O3–Ce5 and Pd/gAl2O3–Ce10, the maximum in activity was reached up to 20 h on stream and then remained constant. In the catalysts with low cerium content and without cerium it can be assumed that the deposition of carbon on palladium becomes important after 20 h of work. However, for the catalysts with high cerium contents, it is observed that it is required a period of induction to reach 100% of conversion. This phenomenon can be related to the formation of the Pd8/PdO species which require a relative narrow abundance to show their oxidant power, which was achieved after 20 h of reaction. The amount of carbon deposited on the catalysts after 30 h of reaction, calculated from TPO measurements, is reported in Fig. 7. It can be observed that the lowest amount of carbon corresponds to the catalysts with the highest Ce content. The fact that the activity does not decay once the maximum CO2
Table 6 Benzene combustion on Pd/g-Al2O3–Ce catalysts with 0, 1, 5, 10 wt% Ce, after 20 h of activation at 250 8C Catalysts
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
Combustion of benzene (%) 100 8C
150 8C
200 8C
250 8C
25 28 33 37
59 69 69 70
88 94 91 92
94 100 100 100
Fig. 7. Amount of carbon deposited on used catalysts after 30 h on stream as a function of the Ce content.
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formation is reached is an evidence that cerium acts as a source of active oxygen which cleans continuously the carbonaceous species deposited on the surface of the catalyst. 4. Conclusions The cerium addition to the aluminum boehmite followed by calcination produces g-Al2O3–Ce supports. Two behaviors can be distinguished in the benzene combustion on the Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts. The Lewis acidic sites on the gAl2O3 and g-Al2O3–Ce1 supports and the dispersion of the Pd catalysts are the responsible of the benzene combustion in catalysts containing 1% Ce and free of Ce. At high Ce contents (5 and 10%), the characteristic oxidation–reduction cycle of cerium serves as oxygen source and it allows Pd8 and PdO to co-exist in a proportion that favors benzene combustion. A period of activation of the catalysts with the reactant mixture at 250 8C for 20 h, leads to the combustion at lower temperatures in the catalysts with cerium. These results are explained by the effect of cerium oxide, which stabilizes Pd in oxidized species, PdO, allowing Pd8 and PdO to co-exist in a proportional ratio, which favors the combustion and inhibits the deposit of carbonaceous species on the Pd surface. Acknowledgements We thank CONACYT for the project SEP-CONACYT2004-C01-46689Q and J.M. Padilla thanks CONACYT for the Postgraduate scholarship. References [1] M.J. Patterson, D.E. Angove, N.W. Cant, Appl. Catal. 26 (2000) 47.
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Catalysis Today 133–135 (2008) 541–547 www.elsevier.com/locate/cattod
Improved Pd/g-Al2O3–Ce catalysts for benzene combustion J.M. Padilla a, G. Del Angel a,*, J. Navarrete b Universidad Auto´noma Metropolitana, Iztapalapa, Departamento de Quı´mica, A´rea de Cata´lisis, Av. San Rafael Atlixco No. 186, C.P. 09340, AP. 55-534, Me´xico, D.F., Mexico b Instituto Mexicano del Petro´leo, Programa de Ingenierı´a Molecular, Me´xico, D.F. 07730, Mexico
a
Available online 1 February 2008
Abstract The catalytic oxidation of benzene on Pd (0.5 wt%)/g-Al2O3–Ce catalysts with 1, 5 and 10 wt% Ce was investigated. The alumina–Ce supports were prepared by adding a Ce-nitrate precursor to an aluminum boehmite. The presence of Lewis acid sites was determined by FTIR-pyridine adsorption spectroscopy. By FTIR-CO adsorption, it was found that the presence of Ce leads to the stabilization of PdO on the supports. The presence of the PdO species on the Pd/g-Al2O3–Ce catalysts was confirmed by X-ray photoelectron spectroscopy (XPS). The combustion of benzene (2600 ppm) in air was carried out at atmospheric pressure. Two behaviors were observed: for the catalysts with low Ce content (1%) and without Ce, the combustion occurs in a broad temperature range 100–280 8C, meanwhile, catalysts with high Ce contents (5, 10%), the benzene combustion occurs in the range 220–280 8C. An activation treatment on Pd/g-Al2O3–Ce catalysts with a reactant mixture at 250 8C for 20 h reduces the temperature of the total combustion to 250 8C. A better stability on Pd/g-Al2O3–Ce catalysts containing 5 and 10% Ce was observed. These results suggest that the oxidative catalytic properties of cerium oxide are shown at temperatures higher than 220 8C, and that an activation treatment is needed to obtain an optimal Pd8/PdO ratio for these species. The stability showed by the catalysts is explained by the effect of the CeO2, which inhibits the deposition of carbonaceous species on the Pd surface. # 2007 Elsevier B.V. All rights reserved. Keywords: Combustion; VOC; Benzene combustion; Pd/g-Al2O3–Ce catalysts; XPS-Pd spectroscopy; FTIR-CO adsorption; FTIR-pyridine adsorption
1. Introduction Air pollution has become one of the most complex environmental problems. Among the most common contamination sources is that produced by volatile organic compounds (VOC). Benzene is one of the main pollutants whose principal sources are petrochemical industries and vehicles [1–4]. The oxidation of the VOCs on a catalyst takes place at temperatures which are lower than those required for thermal destruction [2]. Noble metals such as Pt, Rh and Pd, deposited in conventional supports such as Al2O3, SiO2, etc., have shown to be efficient for this reaction [4]. The Al2O3/CeO2 support is one of the most widely used in three-way automotive catalysts (used for the elimination of the pollutant gases in the exhaust automobile gases). The cerium addition to alumina produces structural changes, improves the dispersion of the metal and it participates
in the stabilization of alumina, avoiding the thermal sinterization. On the other hand, the Ce3+/Ce4+ oxidation–reduction properties make cerium a good promoter for the combustion reaction due to its capacity to deliver oxygen. [5]. The present work is a study of the effect of Ce content on Pd/ g-Al2O3–Ce catalysts for the benzene combustion, as well as the effect of the activation and stabilization of the catalysts after 30 h on stream. The g-Al2O3–Ce supports were obtained from the addition of cerium nitrate to an aluminum boehmite and the characterizations were carried out by BET nitrogen adsorption, Fourier transform infrared spectroscopy of pyridine (FTIR), FTIR-CO adsorption and X-ray photoelectron spectroscopy (XPS). 2. Experimental 2.1. Support preparations
* Corresponding author. E-mail address: [email protected] (G. Del Angel). 0920-5861/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2007.12.053
The g-Al2O3 reference support was obtained by calcination in air of the aluminum boehmite phase (Catapal-B Condea high
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purity 99.999%) at 650 8C. The alumina doped with Ce with 1, 5 and 10 wt%, was prepared by adding Ce(NO3)36H2O (Strem Chemicals 99.9%) to the aluminum boehmite which were maintained in stirring in an aqueous media for 4 h, the solids were dried in an oven at 120 8C for 12 h and then calcined in air flow at 650 8C for 24 h. The supports were labeled as g-Al2O3– CeX, where X represents the wt% of Ce in the support.
equipped with CaF2 windows. The samples were pretreated in situ at 400 8C for 30 min under vacuum (10 6 Torr). The CO admission at 20 Torr was carried out at 200 8C, afterwards the cell was cooled down to room temperature, the excess of CO was evacuated under vacuum during 10 min and then the CO absorption spectra were recorded. 2.7. X-ray photoelectron spectroscopy
2.2. Catalysts preparation The Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts were prepared by impregnation of the supports with a PdCl2 (Strem Chemicals 99.9%) aqueous solution at pH 2 to obtain 0.5 wt% of Pd in all the catalysts. Afterwards the catalysts were calcined at 500 8C during 4 h and then reduced with hydrogen at 500 8C for 5 h. 2.3. Specific surface area, BET The determination of the BET surface area and the pore diameter was carried out in a QuantaChrome Multistation Autosorb 3B analyzer. Before performing the adsorption measurements, the samples were outgassed at 400 8C, in a vacuum of 10 3 Torr for 3 h. The specific surface area was calculated by the BET equation and the pore diameter by the BJH method. 2.4. TPO measurements The TPO study was carried out in a CHEMBET-3000 apparatus using a thermal conductivity detector (TCD), and 0.1 g of catalyst. In these experiments, the flow rate of the 5% O2/95% He mixture was 10 mL/min and the heating rate was 10 8C/min. Finally, the spectra were recorded, for fresh and after-combustion samples, from room temperature to 500 8C, only one peak was observed at around 100 8C. 2.5. FTIR-pyridine adsorption The FTIR spectra for pyridine adsorption were determined with a Nicolet model 170-SX spectrometer. The powdered sample was pressed until a transparent sample was obtained, which was placed in a glass cell equipped with CaF2 windows. The samples were pretreated in a vacuum of 10 6 Torr at 400 8C for 30 min in order to clean the surface. The samples were then cooled down to room temperature. The pyridine contained in a glass bulb coupled to the cell was passed on the catalyst. After the pyridine excess was eliminated under vacuum for 10 min, and then the spectra were registered at different desorption temperatures. The pyridine desorbed at each temperature was eliminated under vacuum (10 6 Torr).
The XPS analyses were carried out in a THERMO VG ESCALAB 250 spectrometer equipped with an aluminum anode (energy of 1486.8 eV) and with an X-ray system monocromator. The X-ray source was powered at 15 kV and 7.5 mA. In order to correct the effect of charge on the XPS spectra, all binding energies were referenced to the C 1s line of adventitious carbon at 284.6 eV. The reduced samples were placed on a thin sheet of indium and then analyzed. In order to control the sample charge in all experiments, an electron flood gun was used. 2.8. Catalytic activity measurements The combustion of benzene in vapor phase was carried out in a conventional flow U-shape glass reactor used in the differential mode at atmospheric pressure. The 2600 ppm of benzene in air were used as reactants in a total flow of 90 cm3/ min. The catalyst (0.1 g) was first treated under air flow (90 cm3/min) at 500 8C for 45 min, afterwards the catalytic system was treated under a flow of a mixture of benzene (2600 ppm)/air at 500 8C during 15 min. Then, the temperature was diminished to 100 8C, under the reactant mixture flow, at this moment the combustion reaction was carried out at different temperatures: 100, 140, 180, 220, 250 and 280 8C. The activity tests were done once a steady state was reached for each temperature. The conversions were measured during this period, where, in the end, almost constant values were reached. To measure the effect of the activation treatment on the catalyst conversions, at the different temperatures, we proceeded in the following way: the sample was treated under a benzene (2600 ppm)/air flow at 250 8C and then maintained on stream during 20 h. Afterwards the combustion reaction proceeded at 100 8C and the temperature was increased by steps of 50 8C until 250 8C. Reactants and products were analyzed by using an on-line gas chromatograph VARIAN 3400 CX equipped with a TCD (thermal conductivity detector) and a FID (flame ionization detector) using a J & W Scientific Megaboro 30 m, with a phase GSQ column that allowed measurements of CO2 and H2O concentrations and a PONA of 50 m columns to follow Benzene conversion. The products detected by the HP-GC–MS 5973 gas chromatograph mass spectrometer were, only, CO2 and H2O. 3. Results and discussion
2.6. FTIR-CO adsorption 3.1. Specific surface area The study of the FTIR-CO adsorption was carried out at room temperature using a Nicolet-FX710 apparatus. The samples pressed in thin wafers were placed in a Pyrex glass cell
The specific surface area of g-Al2O3 obtained from the calcination of the boehmite at 650 8C was around 210 m2/g.
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Table 1 Characterization of the Pd/g-Al2O3 and Pd/g-Al2O3–Ce Catalysts Catalyst
Pd (wt%)
Dispersion (%)
´˚ Particle size (A )
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
0.51 0.49 0.52 0.50
85 91 83 68
12.4 11.6 12.8 15.6
The addition of Ce produced a decrease in the specific surface area. The supports showed specific areas of 153, 145 and 146 m2/g, for g-Al2O3–Ce1, g-Al2O3–Ce5 g-Al2O3–Ce10, respectively. On the other hand, the pore diameter was ˚ ) in all the supports. The lower practically constant (78–94 A specific surface areas of the supports containing cerium indicate that probably the ceria crystallites plugged the aperture of the pores of alumina. However, the formation of Al–O–Ce– bonds cannot be discarded [6]. The content of Pd was determined by atomic absorption spectroscopy and the obtained values are reported in Table 1. The palladium content for all the catalysts varied between 0.49– 0.52 wt%, which was close to the Pd nominal (0.5 wt% Pd). 3.2. FTIR-pyridine adsorption The infrared spectra for the pyridine adsorption showed typical vibration bands of Lewis acid sites at 1590, 1490 and 1445 cm 1 in all the supports. In Fig. 1 are shown the mmol/g of pyridine adsorbed as a function of the desorption temperature for the different catalysts. It is observed a decrease in the number and strength of the Lewis acid sites with the cerium content in the support. The decrease of the g-alumina acidity with cerium addition can be explained by the deposition of Ce particles on the alumina acid sites. 3.3. FTIR-CO adsorption In Fig. 2 is shown the FTIR spectra of the CO adsorption for the different catalysts. The adsorption of CO on the Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts showed the typical absorption bands for CO adsorbed on Pd catalysts [7]. The adsorption band located at 1969 cm 1 in the Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts, has been assigned to symmetric and non-symmetric
Fig. 1. Pyridine adsorbed (mmol/g) as a function of the desorption temperature for the catalysts at different cerium contents (0, 1, 5 and 10%) obtained by FTIR.
Fig. 2. FTIR spectra of CO adsorbed on Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts.
adsorptions of CO in the regions 1910–1980 cm 1 [7]. For catalysts containing 5 and 10% Ce the band was slightly shifted to higher frequencies. On the other hand, the adsorption at around 2073 cm 1 also appeared shifted to higher frequencies in catalysts with 5 and 10% of Ce. The band in the 2085– 2065 cm 1 is assigned to the CO lineal adsorption on the high index crystallographic planes, i.e. adsorption on edges, corner, terraces and kinks [8]. The adsorption band at 2160 cm 1 in catalysts containing 5 and 10 wt% Ce was assigned to the CO– Pd2+ [9]. Palladium in its oxidized state (Pd2+) was only detectable in such samples. Then we can assume that the presence of cerium stabilizes the oxidized PdO phase. 3.4. Dispersion determined by FTIR-CO In this work, information was obtained from the elemental peaks of the CO spectra which were obtained from their deconvolution and the attribution of the vibration bands to determine the dispersion of the Pd catalysts [10,11], similarly, it has been reported in previous works [12]. The deconvolution of the FTIR spectra of the Pd/gAl2O3 catalysts in the elemental bands is shown in Fig. 3. The (a) and (b) peaks corresponded to the regions of 2050–2100 cm 1 and 2030–2050 cm 1 which are, respectively attributed to the discontinuity between the
Fig. 3. Deconvoluted FTIR-CO adsorbed spectrum for the Pd/g-Al2O3 catalyst.
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Fig. 4. Pd 3d5/2 and 3d3/2 core level spectra of Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts.
plans (edges, corner, terraces and kinks) where CO is linearly adsorbed. The (c) peak at 1950–1990 cm 1 was assigned to the 100 planes with bridged CO adsorbed, the domain of 1910– 1940 cm 1 corresponds to the planes 1 1 1 and also to bridged CO adsorbed (d), whereas the peaks in the ranges of 1840– 1880 cm 1 and 1810–1940 cm 1 (d and e) were associated to the interactions metal–support or the interfaces of the metallic support [13]. The dispersion values calculated from the deconvolution of the FTIR-CO adsorption spectra for all the catalysts are reported in Table 1. By the metal dispersion, defined as D (%) = N(CO)surf/N(Pd)total 100 and applying the Beer–Lambert law, we calculated the total number of CO molecules adsorbed on the surface by N(CO)surf = AiS/We, where Ai = integrated absorbance of the total bands obtained by the IR spectra, S = area of the wafer used in IR experiment, W = weight of the catalyst wafer and e = molar extinction coefficient (total) of CO adsorbed for Pd: 2.9 10 7 cm/mol [14]. For the Pd/g-Al2O3 catalyst, the dispersion was 85%, while for the Pd catalysts with 1, 5 and 10 wt% Ce, the dispersions were 91, 83 and 68%, respectively. The catalyst with 1% Ce showed the highest dispersions, this results are in agreement with those reported in the literature, where is wellknown that the presence of Ce at low concentrations favors the dispersion of the metal on Al2O3–CeO2 or La2O3–Al2O3 supports. In the Pd/g-Al2O3–Ce10 catalyst with high load of Ce (10%), the dispersion decreases giving a value of 68%. This dispersion diminution could be explained by the deposition of
cerium oxide on the surface of the metal, blocking some Pd sites [15]. 3.5. XPS spectra characterization The deconvoluted XPS photoelectron spectra for the Pd 3d core level region of the Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts are shown in Fig. 4. The deconvolution of the peaks showed three components for 3d5/2 and for 3d3/2 core levels in Pd/g-Al2O3–Ce catalysts. The binding energies of Pd 3d5/2, Al 2p core levels and the percent of Pd species are reported in Table 2. The binding energy of the Pd 3d5/2 core level for Pd in the pattern catalysts, Pd/g-Al2O3, is 335.7 eV, which corresponds to Pd8 [16]. The peak placed at 337.9 eV in the pattern catalyst Pd/g-Al2O3 has been related to Pd-chlorine species indicating that Pd is in a higher oxidation state likely Pd2+ [17]. In the catalysts containing cerium, the PdO species was observed at 3d5/2 core level with binding energies in the range of 336.3–336.0 eV for all the catalysts. As it can be seen, the Pd8 is present in all the Pd catalysts, however, the signal at 337.9 eV is only observed in the catalyst pattern, whose relative percent of this species, calculated from the deconvoluted peak, is around 21% (Table 2). The PdO was observed in catalysts where Ce was added: in catalysts with 1% Ce this species had 7%, for higher contents of Ce, 5 and 10%, the relative percent of this specie was 50 and 53%, respectively, Table 2. The binding energies corresponding to 337.1–337.3 eV, for Pd/g-Al2O3–Ce, have been assigned to the interaction of small Pd clusters with the support. The interaction of small clusters
J.M. Padilla et al. / Catalysis Today 133–135 (2008) 541–547 Table 2 Data from the XPS analyses of the Pd/g-Al2O3–Ce catalysts Catalyst
Binding energy (eV)
Pd species (%)
Al 2p
Pd/g-Al2O3
335.7 336.1 337.1 337.9
74.5
41 (Pd8) 7 (PdO) 31(Pd/Al2O3) 21 (PdCl2)
Pd/g-Al2O3–Ce1
335.6 336.3 337.2
74.5
32 (Pd8) 28 (PdO) 40 (Pd/Al2O3–Ce, Pd/Al2O3)
335.1 336.0 337.3
74.5
335.2 336.0 337.3
74.5
Pd/g-Al2O3–Ce5
Pd/g-Al2O3–Ce10
Table 3 Atomic percent of the different elements of the Pd/Al2O3-Ce catalysts determined by XPS Catalyst
Pd 3d5/2
34 (Pd8) 50 (PdO) 16 (Pd/Al2O3–Ce) 28 (Pd8) 53 (PdO) 19 (Pd/Al2O3–Ce)
with oxide substrates such as Pd–Al2O3 [18] and Pd–CeO2 has been studied [19], the results showed a shift to a higher binding energy in the Pd–g-Al2O3 and Pd–Ce systems. The typical Ce 3d region spectrum for the Pd/g-Al2O3–Ce10 catalyst is shown in Fig. 5. The characteristic peaks for Ce2O3 and CeO2 with binding energies of 880 and 882.6 eV for Ce3+ and Ce4+ species, respectively are observed. It was also observed a peak which has been associated to the interaction of cerium and palladium CePd3 with the corresponding energy of 884.4 eV [20]. The characteristic Al 2p binding energy of 74.5 eV is observed in all the samples, Table 2. The atomic percent of the different species and the surface atomic ratios calculated for Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts determined by XPS are reported in Tables 3 and 4, respectively. From this information, it is observed that the Ce/Al ratio increased and the Pd/Ce ratio decreased, this suggests that cerium could be segregated on the surface as the cerium content increases. The Cl/Pd ratio is around 2 in Pd/Al2O3 catalyst, which confirms the presence of the PdCl2 in this catalyst, that ratio decreases with the amount of Ce in the catalyst. The Cl/Ce ratio follows the same trend, also decreases with the cerium
545
Atomic%
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
Al
O
Pd
Cl
Ce
41.33 42.30 41.95 41.96
58.45 57.56 57.12 57.13
0.07 0.08 0.16 0.15
0.15 0.06 0.05 0.05
– 0.20 0.71 0.79
Table 4 XPS atomic rations for the Pd/g-Al2O3–Ce catalysts Catalyst
Pd/Al
Ce/Al
Pd/Ce
Cl/Pd
Cl/Ce
Cl/Al
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
0.0017 0.0019 0.0038 0.0036
– 0.0047 0.0169 0.0188
– 0.4000 0.2254 0.1899
2.1429 0.7500 0.3125 0.3333
– 0.3000 0.0704 0.0633
0.0036 0.0014 0.0012 0.0012
load. The Pd/Al2O3 and Pd/Al2O3–Ce1 catalysts show the highest Cl/Al ratios. 3.6. Catalytic activity The catalytic behavior of the Pd/g-Al2O3 and Pd/g-Al2O3– Ce catalysts in the combustion of benzene as a function of the temperature is reported in Table 5. Two different behaviors can be observed, the first presented by the Pd/g-Al2O3, Pd/gAl2O3–Ce1 catalysts and the second by the Pd/g-Al2O3–Ce5 and Pd/g-Al2O3–Ce10 catalysts. In the Pd/alumina catalyst without cerium and with 1% Ce, the benzene combustion occurred in a wide range of temperatures starting from 100 8C and rising to the total conversion, towards CO2, at 280 8C. On the other hand, the catalysts containing 5 and 10% Ce, the combustion of benzene was produced in a low interval of temperatures, 250–280 8C, with selectivity towards CO2 of 100%. We assume that two different mechanisms of benzene oxidation occurred in both cases. In the first case, the behavior of the Pd/g-Al2O3 and Pd/gAl2O3–Ce1 catalysts can be explained by the acidic character of the alumina support. According to the literature the catalysts with supports showing high acidity present a good performance for the combustion reactions [21]. It has been reported that the organic compounds interact with the strong Lewis acid sites which activate the organic compounds [22]. We can assume that both catalysts, without Ce and containing 1% Ce have enough Table 5 Benzene combustion on Pd/g-Al2O3–Ce catalysts with 0, 1, 5, 10 wt% Ce Catalyst
Fig. 5. XPS spectra at Ce 3d region of the Pd/g-Al2O3–Ce10 catalyst, showing the deconvoluted peaks.
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
Combustion of benzene (%) 100 8C
140 8C
180 8C
220 8C
250 8C
280 8C
38 31 0 0
74 35 0 0
78 75 0 0
85 88 0 0
90 97 99 82
100 100 100 100
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acid sites (Fig. 1) in the catalyst to lead the combustion. Another factor that could be playing a role is the metal dispersion. The sensitive structure of the benzene combustion has been pointed out by various authors [23,3]. The Pd/g-Al2O3 and Pd/g-Al2O3– Ce1 catalysts showed a high dispersion of palladium (85 and 91%), then, it cannot be discarded a positive effect because of the small metal particles present in this catalysts. On the other hand, the Pd/g-Al2O3–Ce5 and Pd/g-Al2O3– Ce10 catalysts did not show activity in the range of temperatures of 100–220 8C, however, a shot in the conversion of benzene at 250 8C is observed, with values of 99 and 82%, respectively. In both catalysts the total benzene conversion was obtained at 280 8C. These results can be explained by a promotion effect of the cerium oxide which is reveled only at high temperatures. In the present case, we assume that the presence of Ce at 5 and 10% favors the oxidizing state of Pd to PdO on the support. Evidence of PdO in these catalysts was shown by infrared spectroscopy CO adsorption (Fig. 2) and by XPS results (Fig. 4). It has also been reported that the maximum in the catalytic activity in the combustion is reached when an optimal Pd8/PdO ratio is reached [24]. The no activity showed by the catalysts with 5 and 10% Ce at low temperatures could be due to the fact that cerium requires a high temperature or a long activation time to reach this ratio. 3.7. Activation effect on the catalytic activity In order to study the effect of the activation of the catalysts on the conversion of the benzene combustion, the catalysts were thermally treated with a reactant mixture. The catalysts were maintained on stream under a benzene (2600 ppm)/air flow at 250 8C during 20 h. After this, the benzene combustion was carried out from 100 to 250 8C. The results of the conversion of benzene at different temperatures are reported in Table 6. Combustion is observed in all the catalysts with and without cerium, beginning from 100 8C. This conversion is increased with the increment of the temperature, being slightly higher in the catalysts with cerium. In the Pd/g-Al2O3–Ce catalysts, no effect of the cerium concentration was observed on the conversion, which is very similar for the different samples, at the different temperatures. In the catalysts containing cerium the total conversion was obtained at 250 8C while in the pattern catalyst, Pd/g-Al2O3, the conversion at this temperature was 94%. As it can be observed, the effect of the activation on the catalysts with cerium leads to the activation of the species Pd8/ PdO which allows conversion at lower temperatures.
Fig. 6. Conversion of benzene after 30 h of reaction at 250 8C on Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts.
3.8. Catalytic stability In addition, the presence of cerium could play an important role in the stabilization of the Pd catalysts. In line with the aforementioned, a study of the stability of the catalysts was carried out in samples without activation treatment. The catalysts were maintained under the combustion reaction at 250 8C during 30 h, see Fig. 6. Again, two types of behavior were observed. For the Pd/g-Al2O3 and Pd/g-Al2O3–Ce1 catalysts a decrease in the conversion was observed after 20 h of reaction, whereas in the catalysts Pd/g-Al2O3–Ce5 and Pd/gAl2O3–Ce10, the maximum in activity was reached up to 20 h on stream and then remained constant. In the catalysts with low cerium content and without cerium it can be assumed that the deposition of carbon on palladium becomes important after 20 h of work. However, for the catalysts with high cerium contents, it is observed that it is required a period of induction to reach 100% of conversion. This phenomenon can be related to the formation of the Pd8/PdO species which require a relative narrow abundance to show their oxidant power, which was achieved after 20 h of reaction. The amount of carbon deposited on the catalysts after 30 h of reaction, calculated from TPO measurements, is reported in Fig. 7. It can be observed that the lowest amount of carbon corresponds to the catalysts with the highest Ce content. The fact that the activity does not decay once the maximum CO2
Table 6 Benzene combustion on Pd/g-Al2O3–Ce catalysts with 0, 1, 5, 10 wt% Ce, after 20 h of activation at 250 8C Catalysts
Pd/g-Al2O3 Pd/g-Al2O3–Ce1 Pd/g-Al2O3–Ce5 Pd/g-Al2O3–Ce10
Combustion of benzene (%) 100 8C
150 8C
200 8C
250 8C
25 28 33 37
59 69 69 70
88 94 91 92
94 100 100 100
Fig. 7. Amount of carbon deposited on used catalysts after 30 h on stream as a function of the Ce content.
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formation is reached is an evidence that cerium acts as a source of active oxygen which cleans continuously the carbonaceous species deposited on the surface of the catalyst. 4. Conclusions The cerium addition to the aluminum boehmite followed by calcination produces g-Al2O3–Ce supports. Two behaviors can be distinguished in the benzene combustion on the Pd/g-Al2O3 and Pd/g-Al2O3–Ce catalysts. The Lewis acidic sites on the gAl2O3 and g-Al2O3–Ce1 supports and the dispersion of the Pd catalysts are the responsible of the benzene combustion in catalysts containing 1% Ce and free of Ce. At high Ce contents (5 and 10%), the characteristic oxidation–reduction cycle of cerium serves as oxygen source and it allows Pd8 and PdO to co-exist in a proportion that favors benzene combustion. A period of activation of the catalysts with the reactant mixture at 250 8C for 20 h, leads to the combustion at lower temperatures in the catalysts with cerium. These results are explained by the effect of cerium oxide, which stabilizes Pd in oxidized species, PdO, allowing Pd8 and PdO to co-exist in a proportional ratio, which favors the combustion and inhibits the deposit of carbonaceous species on the Pd surface. Acknowledgements We thank CONACYT for the project SEP-CONACYT2004-C01-46689Q and J.M. Padilla thanks CONACYT for the Postgraduate scholarship. References [1] M.J. Patterson, D.E. Angove, N.W. Cant, Appl. Catal. 26 (2000) 47.
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